An exhaust gas purification system in which a DPD and a SCR (Selective Catalytic Reduction) device are connected to an exhaust pipe has been developed for purifying and discharging exhaust gas from diesel engines.
In such an exhaust gas purification system, PM contained in the exhaust gas is collected by the DPD. Further, in the SCR system provided with a SCR device in such exhaust gas purification system, urea water stored in an urea tank is supplied to the exhaust gas upstream of the SCR, ammonia is generated by the heat of the exhaust gas, and NOx is reduced and decreased on the SCR catalyst by the ammonia (see, for example, Patent Documents 1 and 2).
Since the PM collected by the DPD causes clogging of the filter, regeneration should be performed by oxidizing, as appropriate, and removing the collected and deposited PM.
In the case where the detection of clogging is performed automatically by an ECU (Engine Control Unit) or manually when the difference between the pressure detected by exhaust gas pressure sensors before and after the DPD reaches an upper limit value, a DPD alarm lamp provided inside a cabin is lit up and the driver starts the DPD regeneration by pushing a regeneration execution switch.
The DPD is constituted by a DOC (Diesel Oxidation Catalyst) constituted by an active catalyst that oxidizes the unburned fuel and a CSF (Catalyzed Soot Filter) that collects the PM contained in the exhaust gas.
The DPD is regenerated by performing multi-injection (pilot injection, pre-injection, main injection, after-injection) of fuel to raise the exhaust gas temperature to a temperature equal to or higher than the catalyst activation temperature of the DOC, then adding post-injection, raising the exhaust gas temperature to a temperature equal to or higher than 500° C., and removing the PM collected on the CSF by burning with such high-temperature exhaust gas. However, where post-injection is performed, fuel oil mixes with the lubricating oil of cylinders, thereby diluting the lubricating oil. Therefore, the DPD regeneration is performed by the so-called exhaust pipe injection in which fuel (HC) is injected in the exhaust pipe upstream of the DPD.
In the exhaust pipe injection, the DPD regeneration is performed in an automatic mode as the vehicle travels and in a manual mode by idle rotation after the vehicle has been stopped, in the same manner as in the post-injection. Usually the automatic regeneration is performed as the vehicle travels, but because the vehicle travels during the regeneration, the exhaust gas temperature is unstable and PM burning residue easily appears inside the DPD when the vehicle is repeatedly accelerated and decelerated. Accordingly, when the regeneration interval is short and the automatic regeneration of the DPD is performed repeatedly, the driver is encouraged to perform manual regeneration, and the driver removes the PM present in the DPD by manual regeneration by pushing the manual regeneration execution switch after the vehicle has been stopped.
Further, when the vehicle is stopped during traveling automatic regeneration, the regeneration is continued by closing an exhaust gas brake valve to prevent the exhaust gas temperature from decreasing, so as to enable the regeneration also by idle rotation.
In such automatic regeneration, the temperature of the exhaust gas flowing into the CSF is detected with an exhaust gas temperature sensor provided downstream of the DOC, a difference between this exhaust gas temperature and a target regeneration temperature is determined, and the exhaust pipe injection amount is proportional integral derivative (PID) controlled based on this difference so as to obtain the target regeneration temperature. Here, P is a proportional control term, I is an integral control term, and D is a derivative control term. In the proportional control term (P term), the operation amount is changed proportionally to the difference, in the integral control term (I term), the differences are added up and the operation amount is changed proportionally to the value obtained, thereby eliminating the residual difference (stationary difference) present in the proportional control, and in the derivative control term (D term), the variation rate of the difference is converted into a speed, and the operation amount proportional thereto is outputted, thereby increasing the response rate and ensuring rapid convergence to the set value.